Electrostatic powder application



June 1967 J. c. BARF-0RD ETAL 3,323,933.

ELECTROSTAT I C POWDER APPL I C AT ION Filed June 21, 1963 4b ummmmmmnnmun l'lllllvl ll l'lllllllllllllll \2 Patented June 6, 1967 3,323,933 ELECTRGSTATIC PQWDER APPLTCATIGN John Cowper Barfcrd, Peter Francis Dias, and John Eavid Glentworth, Doncaster, England, assignors to Societe Anonyme de Machines Electrostatiques, Grenoble, Isere, France, a corporation of France Filed June 21, 1963, Ser. No. 289,651 Claims priority, application Great Britain, June 22, 1962, 24,063/ 62 15 Claims. (Cl. 117-47) This invention relates to the coating of objects with a powder material, such as a synthetic plastic or ceramic, more particularly for forming a coherent or continuous layer of the material on the surface of an object.

The invention consists in a method of coating an object with a powdered material which comprises breakingup or dispersing the powder mechanically in the vicinity of one or more charging electrodes maintained at a poten tial difference with respect to the object, whereby said dispersed powder particles are electrostatically charged and attracted on to the object to be coated. In order to form the continuous or coherent layer the powder particles adhering to the object may be fused or consolidated together by heating or in any other convenient manner.

According to a feature of the invention the powder material is dispersed by moving the charging electrodes in or through the powder. Preferably, the charging electrodes comprise rotors or paddle-wheel members formed with points or sharp edges in order to produce a high charge density in the vicinity of said points or edges through which the powder particles will pass upon being mechanically dispersed by the rotors. The rotors may disperse the powder particles by flinging them into the atmosphere above the powder layer.

The invention also consists in apparatus for carrying out the process, comprising a container, trough or other support for the powder material and one or more charging electrodes which are moved relative to and come into contact with the powder to disperse and charge the same. The mechanical action of the charging electrodes on the powder breaks up lumps or agglomerations of the powder and ensures that the individual powder partirles will be charged and attracted towards the object.

The object to be coated, which may be electrically conducting is preferably positioned above the powder and held at earth potential to produce an electric field between the charging electrodes and the object by which the charged particles will be conveyed to and deposited on the object.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawing, in which:

FIGURE 1 is a diagrammatic section of an arrangement according to the invention.

FIGURE 2 shows three forms of blades which may be used in the arrangement of FIGURE 1.

FIGURE 3 is a view, corresponding to FIGURE 1, of a modified arrangement.

FIGURE 4 is a diagrammatic perspective view of an alternative arrangement for mechanically breaking up and charging the powder.

FIGURE 5 is a section of another arrangement according to the invention.

FIGURE 1 shows a trough 1, preferably made from a non-conducting material, in which is/are arranged one or more rotors 2, which may be similar to paddle-wheels. The trough is filled with suflicient powder 3 partially or completely to cover the rotors so that the rotors tend to fling the powder up in the air to a degree which will depend upon their geometry and speed of rotation. The rotors are conveniently constructed from a good electrical conducting material, and are connected to a source of high potential it The rotors are designed so that there is an area of maximum charge density at their outer edges by forming these as blades or spikes, it being well known that the charge density is greatest at a point or sharp edge. Side views of three suggested designs of rotor with blades 4a or 4c or spikes 4b are shown in FIGURE 2.

The object 5 to be coated, for example a wire or strip, is passed over the trough and is maintained at earth potential so that an electrical field is set up between the rotors charged to a high positive or negative potential and the object at earth potential. The powder is mechanicaily dispersed by the rotors, and the action of the rotors ensures that all, or a majority, of the powder passes through the area of discharge or corona at the edges of the blades, so that the powder particles are charged as they are dispersed by the rotors. The charged powder is electrostatically attracted to the object to be coated. A certain amount of the powder will tend to overshoot the object and adhere to its upper surface. If the object is a wire or mesh or some open structure both sides will be coated at the same time as the object is continuously advanced above the trough. The uniformity of the coating may be enhanced by vibrating the object gently to remove excess powder which may settle on its upper surfaces. The charged powder is Sllfil'ClfiHtlY adherent to allow for the object to be handled and passed into an oven to fuse or sinter the powder coating to produce a continuous coating.

In the apparatus shown in FIGURE 1 the rotors may be revolved in any desired direction and are preferably driven and synchronised by gears made of electrically insulating material, to ensure that the high voltage is concentrated at the edges of the blades where maximum discharge is required and not dissipated at the ends of the rotors or the shafts or the means of driving the rotors.

The level of powder along the length of the trough should be kept reasonably even, such as by vibration, and various methods of feeding the powder into the trough may be employed. FIGURE 3 shows a suggested method in which powder is fed into opposite sides of the trough beneath shields 11 of insulating material.

According to a modification, the breaking-up or dispersion of the powder may be effected by a number of point or edge sources of discharge submerged in a bed of the powder and moving in the powder. FIGURE 4 shows a typical example where a number of combs 12 may be attached to two forked frameworks 13 in such a way that they move in opposite directions in the powder, thereby breaking up and charging the powder. It will be appreciated that this is not an ideal method since channelling can occur in the powder and portions of the powder may not be charged.

In another modification, the rotors may move over a stationary bed of powder at the same time as they revolve or, alternatively, a bed of powder may be moved, for example on a conveyor belt, with the rotating rotors in a stationary position beneath which the powder moves. Alternatively, a combination of moving rotors and moving bed may be employed. Vibration of the powder may also facilitate its movement and dispersion.

A further modification is shown in FIGURE 5, where a cylindrical drum 6 containing a quantity of powder 3 is caused to revolve around an object 5. Blades or points of discharge 7, which are maintained at a high potential are located on the inside of the drum and carry up a quantity of the powder to a position where it is allowed to fall through the area of discharge and to be attracted on to the object. Some sort of automatic switching device may be provided so that only the two or three blades which are actually carrying up the powder, would be charged to the appropriate potential.

Materials which can be used for coating by this method include thermoplastics and thermosetting plastics, glass or ceramic materials, such substances being capable of transformation into a powdery state and melting or sintering within a temperature range below the melting point of the article to be coated. Examples of the more useful plastic powders include polyethylene, polyvinyl chloride, polyamides, cellulosic plastics, and epoxy resins. Other powders, such as adhesives, insecticides, fungicides etc. for spraying of crops, abrasives, and flocks, for coat- 'ing textiles and papers, can also be applied by this method. When the powdered materials used are nonconductors of electricity, the charge is formed by depositing on the surface of the powder particles charged ions which are generated by the discharge from the high potential source. To achieve this deposit the powder must touch the charging electrode and pass through the area of discharge (corona), and if each particle is to be charged the particles must be reasonably discrete. If a lump or an agglomerate of powder passes through the area of discharge, the particles buried in the middle of the agglomerate are unlikely to have ions deposited on their surface. The breaking-up or dispersing of the particles to a sufiicient degree at the time they pass through the area of discharge is therefore important for efiicient charging of the particles.

The high voltage electrostatic source may be provided by any convenient electrostatic generator or power pack.

The object to be coated should be a sufficiently good conductor to allow for earthing. Metals, of course, are particularly suitable, but other materials, for example porous or fibrous materials such as wood, asbestos, fabrics, etc, which usually contain sufficient moisture to form a satisfactory earth, may be coated with an electrostatically charged powder as herein described.

Whilst particular embodiments have been described, it will be understood that various modifications may be made without departing from the scope of the invention, for example the object may be heated before it is passed over the trough so that the powder particles which are attracted thereto and deposited thereon will be partially or completely fused by the heat of the object.

We claim:

1. A method of coating an object with a powder material, which comprises mechanically dispersing the powder by means of a rotating charging electrode formed with discharge edges in the vicinity of which the powder particles are caused to move by the mechanical dispersing :action of said electrode, said charging electrode breaking up agglomerations of the powder in the vicinity of the charging electrode, and maintaining said charging electrode at a potential diflerence with respect to the object, said electrode electrostatically charging said dis persed powder particles, the charged particles being attracted on to the object.

2. Method as claimed in claim 1, which further comprises converting the powder coating on the object into .a coherent layer by heating.

3. A method of coating an object with a powder mate- :rial, which comprises mechanically dispersing a body of :solid powder particles by moving a charging electrode through said body, said charging electrode breaking up agglomerations of the powder during its movement and flinging the dispersed particles upwardly from said body, and maintaining said charging electrode at a potential difference with respect to the object to electrostatically attract the mechanically dispersed particles thereto.

4. A method of coating an object with a powder material, which comprises mechanically dispersing a body of solid powder particles by moving a charging electrode through said body, said charging electrode flinging the dispersed particles upwardly from said body and including at least one sharp discharge edge, and maintaining said discharge edge at a potential difference with respect to the object to electrostatically attract the mechanically dispersed particles thereto.

5. Apparatus for coating an object with a powder material, comprising a support for a layer of the powder material, at least one charging electrode having a movable discharge edge extending in a straight line for an appreciable distance in a direction transverse to the direction of movement thereof, means for moving the discharge edge relative to and through the powder layer, the relative movement of said discharge edge flinging particles of said powder upwardly from said layer to mechanically disperse the same, and means for applying a DC. potential to said electrode to charge the powder particles dispersed by the electrode.

6. Apparatus as claimed in claim 5, wherein the powder is contained in a rotatable drum having inwardly projecting charging blades on its internal surface, said charging blades dispersing the powder particles into the interior of the drum as it rotates.

7. Apparatus as claimed in claim 5, wherein said at least one charging electrode comprises a rotor which rotates in contact with the powder material.

8. Apparatus as claimed in claim 7, wherein the powder is contained in a trough in which said at least one rotor is mounted, and means for rotating said at least one rotor at a speed sufficient to fling the powder particles above the normal level of the powder layer in the trough.

9. Apparatus as claimed in claim 8, wherein said at least one rotor has blades formed with relatively sharp edges.

19. Apparatus of claimed in claim 8, including means for supporting the object to be coated above the powder layer, and means for maintaining said object at a potential different from that of said at least one charging elec-' trode.

11. Apparatus for coating an object with a powder material, comprising a support for a layer of the powder material, electrostatic charging means disposed at least partly within said layer of material, means for moving said charging means relative to said layer to fling individual powder particles upwardly away from said layer and thereby mechanically disperse the same, said charging means having a discharge edge extending in a straight line for an appreciable distance in a direction transverse to the direction of movement of said charging means, and means for establishing a charging potential between said charging means and the object to electrostatically attrack the mechanically dispersed particles thereto.

12. Apparatus for coating an object with a powder material, comprising trough means for a supporting a bed of powder material to be applied to said object, said material having an upper surface within said trough means, means for positioning said object in spaced relationship with said upper surface, electrostatic charging means disposed in said bed at least partly beneath said upper surface, means for moving said charging means relative to said material to fling individual powder particles upwardly away from said surface and thereby mechanically disperse the same, said charging means having a discharge edge extending in a straight line for an appreciable distance in a direction transverse to the direction of movement of said charging means, and means for establishing a charging potential between said charging means and the object to electrostatically attract the mechani cally dispersed particles thereto.

13. Apparatus for coating an object with a powder material, comprising trough means for supporting a body of the powder material to be applied to said object, said material having an exposed free surface, means .for positioning said object in spaced relationship with said surface, rotary electrostatic charging means in contact with said body of material, said charging means including a plurality of radial charging electrodes extending at least partly within said body, each of said charging electrodes having a discharge edge extending in a straight line for an appreciable distance in a direction transverse to the axis of rotation of said charging means, means for rotating said charging means to move said electrodes relative to said mass of material and thereby fling individual powder particles upwardly away from said surface, said electrodes breaking up agglomerations of particles within said body and mechanically dispersing said particles, and means for establishing a charging potential between said electrodes and the object to electrostatically attract the mechanically dispersed particles thereto.

14. Apparatus for coating an object with a powder material, comprising trough means for supporting a mass of the powder material, said mass of material having a substantially horizontal free upper surface, means for positioning said object above said upper surface in spaced relationship, therewith, rotary electrostatic charging means disposed in said trough means in contact with said mass of material, said charging means including a plurality of paddle-like members extending at least partly within said mass for movement therethrough, means for rotating said charging means to move said members through said mass of material and thereby fling individual powder particles upwardly away from said surface to mechanically disperse the same, and means for establishing a charging potential between said paddle-like members and the object to electrostatically attract the mechanically dispersed particles thereto.

15. Apparatus for coating an object with a powder material, comprising a trough for supporting a layer of the powder material, at least one rotary charging electrode mounted for rotation in contact with the powder material, said rotary charging electrode being formed with spikes which project outwardly from the axis of rotation thereof, means for rotating said rotary charging electrode relative to and through the powder layer at a speed suflicient to fling the powder particles above the normal level of the powder in said trough, the relative movement of said rotary charging electrode mechanically dispersing said powder, and means for applying a DC. potential to said rotary charging electrode to charge the powder particles dispersed :by the electrode.

References Cited UNITED STATES PATENTS 2,675,330 4/1954 Schwartz et al 1l7-17 2,748,018 5 6 Miller 1l86 26 X 2,777,784 1/ 1957 Miller 11793.42 2,820,716 1/1958 Harmon et a1 118- 636 X ALFRED L. LEAVITT, Primary Examiner.

MURRAY KATZ, Examiner.

A. GOLIAN, Assistant Examiner. 

1. A METHOD OF COATING AN OBJECT WITH A POWDER MATEERIAL, WHICH COMPRISES MECHANICALLY DISPERSING THE POWDER BY MEANS OF A ROTATING CHARGING ELECTRODE FORMED WITH DISCHARGE EDGES IN THE VICINITY OF WHICH THE POWDER PARTICLES ARE CAUSED TO MOVE BY THE MECHANICAL DISPERSING ACTION OF SAID ELECTRODE, SAID CHARGING ELECTRODE BREAKING UP AGGLOMERATIONS OF THE POWDER IN THE VICINITY OF THE CHARGING ELECTRODE, AND MAINTAINING SAID CHARGING ELECTRODE AT A POTENTIAL DIFFERENCE WITH RESPECT TO THE OBJECT, SAID ELECTRODE ELECTROSTATICALLY CHARGING SAID DISPERSED POWDER PARTICLES, THE CHARGED PARTICLES BEING ATTRACTED ON TO THE OBJECT. 